1. Field of the Invention
The present invention relates to a substrate holding apparatus or a mask stage base in an exposure apparatus such as an X-ray exposure apparatus in which synchrotron radiation light is utilized as an exposure light, and particularly, to a substrate holding apparatus for vertically holding a substrate or a mask by a magnetic attracting force.
2. Related Background Art
In recent years, there has been a growing need to improve refining techniques in semiconductor-device fabricating apparatuses especially as capacity of semiconductor memories becomes greater and greater.
An X-ray exposure apparatus in which synchrotron radiation light is utilized as an exposure light has been developed as a means for improving the refining such as X-ray exposure apparatus, a wafer and mask are vertically held and exposed to the exposure light, differently from conventional exposure apparatuses which use, for example, far ultra-violet rays as an exposure light (see Japanese patent application unexamined publication No. 2-100311).
There are a vacuum-attraction system, a magnetic-attraction system and so forth as a mask holding apparatus for attracting and holding the mask. In the vacuum-attraction system, however, there is the problem that the mask drops when troubles occur in a vacuum source and the like because the mask is vertically held in the X-ray exposure apparatus. In contrast, in the magnetic-attraction system, there is the advantage that dropping of the mask is prevented because the mask is attracted by a magnetic force of a permanent magnet.
FIG. 1 shows an example of prior art mask holding apparatuses of the magnetic-attraction system.
The mask holding apparatus includes an annular mask stage 115. The shape of its cross section is square or rectangular. A chuck surface (when a mask 101 is attracted) of the mask stage 115 is divided into an inner peripheral portion and an outer peripheral portion by a continuously extending groove 137 whose cross section has a rectangular shape. The mask holding apparatus further includes a magnetic ring 116 fitted into the groove 137.
The inner portion of the mask stage 115 is flatly lapped with a high precision to be a close tolerance flat surface 136 (e.g., flatness is below 1.mu. and roughness is below 0.08.mu.). Thus, a high degree of flatness of the attracted mask 101 is obtained. Further, eight magnetic units 140.sub.1 -140.sub.8 (the magnetic unit 140.sub.8 is not shown) are disposed under the magnetic ring 116, and the mask 101 is attracted and held by magnetically attracting a magnetic ring 133 embedded in the mask 101 by the magnetic units 140.sub.1 -140.sub.8.
As shown in FIG. 2, the magnetic unit 140.sub.1 includes three yokes 141.sub.1, 141.sub.2 and 141.sub.3, a permanent magnet 142, a coil 143.sub.1 wound on the yoke 141.sub.1 and a coil 143.sub.2 wound on the yoke 141.sub.2. The magnetic unit 140.sub.1 takes an attracting state in which the magnetic force of the permanent magnet 142 is effective when no current flows in the two coils 143.sub.1 and 143.sub.2. In contrast, the magnetic force of the permanent magnet 142 is eliminated by magnetic forces generated by the coils 143.sub.1 and 143.sub.2 to be ineffective when current flows in each of the coils 143.sub.1 and 143.sub.2. The same is true with the other magnetic units 140.sub.2 -140.sub.8.
In the prior art mask holding apparatus, however, current should continuously flow in each of the coils 143.sub.1 and 143.sub.2 to maintain each of the magnetic units 140.sub.1 -140.sub.8 in the ineffective state during a positioning operation period (e.g., about 2-3 seconds) in which the mask holding apparatus receives the mask 101 from a mask transporting means such as a mask hand. As a result, heat generated by the current flow is conducted to the mask 101 to generate thermal strains in the mask 101, leading to the degradation of exposure accuracy.